The present invention relates to a method of improving tensile residual stress in a circumferential weld zone for connecting a piping or in a circumferential weld zone of a cylindrical vessel and, more particularly, to a method of improving residual stress in a circumferential weld zone of two members one of which has a high rigidity.
Generally, tensile residual stress in the order of yield stress of the material being used exists in the inner and outer surfaces of weld zone, with the tensile residual stress causing fatigue failure, stress corrosion or the like to the weld zone of the vessel or piping depending on the condition in use of the apparatus, resulting in a low availability factor of the apparatus. The stress corrosion cracking caused in the weld zone of piping of a stainless steel SUS 304 in the primary cooling system piping of a nuclear power generation plant with boiling water reactor is typically susceptible to tension residual stress resulting in decreasing the availability factor of power generation plant. In order to prevent stress corrosion cracking, various studies have been made to develop methods for reducing the tensile residual stress in the weld zone of piping. As a result, a stress-improving method using a high-frequency induction heating was developed as a counter measure method for an already existing and operating plant and has been used in many actual plants. This method comprises the steps of providing around the outer periphery of the annular circumferential weld zone of pipes a high-frequency induction heating coil for local heating, and rapidly heating the weld zone through a high-frequency induction heating device and a cable while cooling the inner surface of pipes by a coolant. By this rapid heating, such a temperature difference occurs in the direction of wall thickness that the temperature at the outer surface of the weld zone becomes high and that at the inner surface of the same becomes low. Consequently, a tensile stress occurs in the inner surface due to the heating, and when a temperature differential between the inner and outer surfaces of piping is about 200.degree. C., a tensile yield occurs in the inner surface thereby causing a tensile permanent set. When the heating treatment is finished and the temperature differential between the inner and outer surfaces is zero, a compressive stress occurs corresponding to the tensile permanent set brought about by the heating. Namely, the residual stress in the inner surface of the pipe which was at a tensile stress before being heated can be relieved.
However, in the case where one of two members in the circumferential weld zone has a higher rigidity than the other, the stress improvement in the inner surface cannot be obtained.
Accordingly, an object of the invention resides in providing a method of improving the residual stress, which can relieve the tensile residual stress in a circumferential weld zone between two members one of which was a higher rigidity than the other, and efficiently using stress distribution characteristics in a local bending deformation.
Another object of the invention resides providing a method of improving residual stress, which can enhance the strength against cracking and the strength against crack development in apparatus and pipe system which are used in a corrosive environment and under an action of a static or dynamic, outer load.
According to advantageous features of the invention, when outwardly concentrated loads are radially applied to the cross-section of a hollow cylinder, a local expanding deformation is caused in the inner surface of the hollow cylinder and local bending stress distributions occur therein.